The present invention generally relates to electrically conductive cermet materials. More particularly, the invention relates to electrically conducting cermet materials suitable for use in end caps for high intensity lamp applications.
High intensity discharge lamps are required to run at high temperatures and high pressures in order to raise the color rendering effect of the lamp and to improve the efficiency of the lamp. Because of operational limitations, various parts of these lamps are made of different types of materials. Bonding of dissimilar materials in high temperature lamps poses numerous challenges such as thermal stresses and cracks that develop because of thermo-mechanical stresses resulting from a mismatch in the thermal coefficients of expansion of the adjoining parts. Ideally, all the materials used in such lamps should have the same coefficient of thermal expansion. If these materials have substantially different coefficients of thermal expansion, at elevated temperatures, stresses develop as the different materials expand at different rates. Articles that are well designed, however, can tolerate some differences in coefficients of thermal expansion.
The components of a high intensity discharge lamp assembly include ceramic envelope, electrodes, end caps, and wire feedthrough conductors. Usually, a ceramic envelope for high intensity lamps is made of alumina or yttrium aluminum garnet (YAG), electrodes are made of refractory metals, and the end caps are usually made of a ceramic metal composite known as cermet. Alumina and YAG both have coefficients of thermal expansion significantly greater than the refractory metal, such as tungsten or molybdenum, which is typically used as electrode.
There have been some efforts to tailor the coefficient of thermal expansion for end cap materials so as to achieve a coefficient of thermal expansion close to that of the ceramic envelope material. In one example, alumina metal cermets (using tungsten or molybdenum as the metal) have been used as end cap materials. But these cermets have limited flexibility to tailor the coefficient of thermal expansion to those of alumina because, as molybdenum or tungsten is added, the coefficient of thermal expansion of the cermet is reduced with respect to that of alumina or YAG. On the other hand, efforts to reduce the molybdenum volume fraction below 0.5 results in lower electrical conductivity and lower ability to weld metallic components to the cermet.
Therefore, there is a need for a cermet material with acceptable electrical conductivity and a coefficient of thermal expansion equivalent to that of alumina or YAG.